This project aims to develop a novel, tissue-equivalent gel-dosimeter of ionizing radiation, which is capable of producing highly-resolved and accurate data on three dimensional dose distributions using optical tomographic densitometry. The dose-response mechanism relies on the production of Rayleigh scattering micro-particles in the gel at the site of radiation absorption, where acrylic comonomers dispersed in the gel are polymerized. Absorption and scattering of collimated light passing through the optically turbid medium produces a net attenuation of light intensity that can be measured and is directly related to the dose. Measurements of the projections of the optical density into different directions can be used to reconstruct the distribution of radiation doses within the gel. Doses of the order of 10 Gy can be measured with a spatial resolution <1mm. In previous work, the MRI properties of the gel have been investigated and used very successfully to create 3D dose maps, but there are significant advantages to the use of a simpler optical method of reading the dose distribution stored by the gel. The gel will find widespread use in radiation therapy practice, for 3D measurements in homogeneous and anthropomorphic phantoms, confirmation of computerized treatment planning and in quality assurance procedures. The goals of Phase I are to devise and evaluate a protocol for convenient and inexpensive preparation of a prototype polymer-gel dosimeter optimized for optical densitometry, to demonstrated its stability and reproducibility as well as spatial uniformity of the dose-response, and to evaluate the feasibility of constructing a low-cost bench top optical scanner for the reconstruction of 3D dose distributions. These studies will prepare the ground for the development of more efficient polymer-gels and an optical reader in Phase II. PROPOSED COMMERCIAL APPLICATION: Estimated 5000 polymer-gels could be distributed annually. This estimate is based upon (a) 30 new accelerators per year, and 40 X-ray and electron beam field; (b) over 3000 existing treatment machines will use 10 gels per year for dosimetry; (c ) research centers might use 5 gels annually for non-routine dose distributions. The annual sales of the gels alone could produce $1,000,000 sales. In addition, if the optical device is available for a cost of $50,000, then 20 such devices per year could be sold, comparable to the number of "water-tank" systems that are in current use.